I used to do some work for a dealer who sold ATC speakers but don't know the difference between these versions. I believe the 20s just underwent a price drop, which addresses for me it's most serious shortcoming.
As far as sound, ATCs are really in a league by themselves. The only question is, do you like the league. ATC is a professional monitor based company that is pushing more into the consumer market, so if you're looking for an "audiophile" sound with holographic imaging and a deep soundstage you may be disappointed as they do tend to have a more forward sound due to their professional roots. That said, as far as tonal accuracy, dynamics, and speed they are absolutely top notch. The ATC midrange is outrageous, and once you hear it you might not want anything else(I know several people that's happened to). My only knock is the top end can be just a bit polite sounding(it is a British speaker afterall) and lacks the last bit of air in the top octaves, but it is still very detailed. Bass is more than fine for a speaker of this size.
Haven't heard the Dynaudios, but ATC blows B&W out of the water by virtue of its ultra clean and powerful midrange. I'd take the 20s over the 804s or 803s(and certainly over the 805s) in a heartbeat as B&W midranges sound relatively dry to my ears and the metal tweeters draw too much attention to themselves on too much music(the dealer also sold B&W so I had lots of time to compare), but they are both quality companies and your personal tastes may prove different. There's no getting around going out a listening.
The only other speaker I'd recommend would be the Talon Khite, which keeps up with the ATC in many areas but adds a bit more finesse in presenting the overall soundstage(i.e. not so forward and disappear better in the room).
By the way, if you want more info. on ATC I'd recommend calling Chris Barry at Avant-Garde Home Entertainment in Chicago. He's a nice guy who is extremely knowledgeable about ATC and will give you the straight poop--(773) 348-5466(FYI, I don't have any affiliation with this dealership, I just know they'll give you lots of good info. on ATC and B&W). Hope this helps and best of luck.
Tim
